The Philippines independent human rights watchdog said Thursday it will investigate President Rodrigo Duterte's admission he killed three criminals years ago, after the United Nations' rights chief called for a murder probe.

Duterte, who is waging an anti-drugs war that has left thousands dead, said last week that he helped police kill three suspected kidnappers early in the first of his several terms as mayor of the southern city of Davao.

Commission on Human Rights chief Jose Gascon said he has formed a team of investigators to look into the matter, even as Duterte's spokesman Ernesto Abella dismissed the UN call as mere "opinion".

"Law enforcement agencies... must investigate as a matter of course any information that suggests that a crime may have been committed with the view to ensuring that perpetrators are ultimately held accountable should the evidence warrant it," Gascon said in a statement.

The commission is an independent government body that prosecutes law enforcers or other officials who commit torture, extrajudicial killings or violate Filipinos' constitutional rights.

The commission had investigated then Davao mayor Duterte over allegations he ran death squads that killed more than a thousand petty criminals there.

Duterte has variously denied or confirmed the allegations. The commission did not file any criminal charges after completing its inquiry.

Gascon said his agency has "reconstituted a team to further investigate (Davao death squads) to look into the new revelations and public admissions that may shed light on our previous findings."

"The team will look into any matter that may further shed light on the killings in Davao that was the subject matter of our previous investigation."

Duterte easily won presidential elections in May largely on a promise to eradicate illegal drugs in society by by launching an unprecedented campaign in which tens of thousands of people would be killed.

More than 5,300 people have died since he took office in late June, including 2,124 at the hands of police. The commission has said it is investigating several cases where police were responsible.

Duterte insists police have not violated any law in killing drug suspects.

On Wednesday Abella said Duterte's admission about the killing of three people referred to "legitimate police action" but did not address the fact the then mayor was not a police officer.

Duterte has said he routinely carried a gun during his early years as mayor of Davao to protect himself from crime. He has not said if the weapon used in the ambush was licensed.